The alkylation of aromatic hydrocarbons such as benzene using solid catalyst is a well-developed art which is practiced commercially in large scale industrial units. One commercial application of this process is the alkylation of benzene with ethylene to produce ethylbenzene, which is subsequently used to produce styrene. The basic configuration, general design, and operation of a catalytic alkylation process to produce ethylbenzene are known in the art. Benzene and ethylene gas pass to an alkylation zone containing generally two or more, and typically from two to six, beds of catalyst in series. Suitable cooling means may be provided between catalyst beds to compensate for the net exothermic heat of reaction in each of the catalyst beds. The net effluent of the beds passes to a separation zone, usually comprising one or more distillation columns, which recover ethylbenzene while recycling benzene and rejecting by-products.
The performance of ethylbenzene processes is influenced by the selectivity and activity of the catalyst in the operating environment of the process. Currently available catalysts for the alkylation of benzene with ethylene include aluminum chloride catalysts and zeolite-containing catalysts. Zeolitic catalysts have certain advantages, such as fewer problems with corrosion and with disposal of spent catalyst, over aluminum chloride catalysts. But, despite these clear advantages, zeolitic catalysts generally produce the same amount of polyethylated benzene by-products as aluminum chloride catalysts. Thus, the goal of minimizing the formation of polyethylbenzenes by maximizing the efficient use of ethylene is a driving force for further developments in zeolitic catalysis for ethylbenzene processes.
The formation of some polyethylbenzenes, such as diethylbenzenes and triethylbenzenes, does not necessarily, however, represent a reduction in the efficient use of ethylene, because diethylbenzenes and triethylbenzenes each can be readily transalkylated by benzene to produce ethylbenzene. In contrast to diethylbenzenes and triethylbenzenes, however, other by-products, which are collectively referred to as flux oil, do represent a reduction in ethylene utilization and a loss of ethylene.
Flux oil is a recognized term in the art of aromatic alkylation. Flux oil is an alkylation by-product that is generally formed by polyalkylation of benzene or by alkylation of benzene with substituted benzene by-products. As such, flux oil may have a variety of different compositions and physical properties depending on the reactants charged to the alkylation process, the alkylation catalyst, the alkylation operating conditions, etc. The composition and properties of the flux oil will also depend on whether a separate transalkylation zone is present, and if such a transalkylation zone is present, on the transalkylation catalyst, the transalkylation conditions, etc.
Flux oil is the highest-boiling fraction of the alkylation reactor effluent that is not recycled to either alkylation or transalkylation in order to produce the desired product. Flux oil is usually obtained by distilling from the alkylation reactor effluent the desired product and all of the recyclable by-products. The low end of the boiling range of flux oil is normally the boiling point of the lightest by-product that does not readily alkylate or transalkylate to produce the desired product. Thus, flux oil contains by-products that do not readily alkylate or transalkylate to produce the desired product, as well as other by-products that can alkylate or transalkylate to produce the desired product but which can not be readily separated by distillation from by-products that do not readily alkylate or transalkylate to produce the desired product. As used herein, components that can not be readily separated by distillation are components that have distillation points that differ by less than 10.degree. F. (5.degree. C.). For example, in ethylbenzene processes flux oil generally includes 1,1-diphenylethane (1,1-DPE) and alkylated 1,1-diphenylethanes which can not be converted to ethylbenzene by transalkylation. But, flux oil in ethylbenzene processes also generally includes heavy polyethylbenzenes, such as tetraethylbenzenes, pentaethylbenzene, and hexaethylbenzene, which can transalkylate to ethylbenzene but which can not be readily separated by distillation from 1,1-diphenylethane or alkylated derivatives thereof. In fact, the by-production of the heavier polyethylbenzenes other than diethylbenzenes and triethylbenzenes and of 1,1-DPE, which are major components of flux oil in ethylbenzene processes, represents virtually all of the reduction in ethylene utilization.
Thus, in order to maximize ethylene utilization and minimize ethylene losses in the production of ethylbenzene, there is a need for a process that minimizes the production of flux oil.